Bring a saucepan of lightly salted water to the boil, ready for the pasta. Wrap the walnuts in a clean tea towel and roughly crush them with a rolling pin.

Add the pasta to the pan of boiling water and cook according to the instructions on the packet, leaving the pasta cooked but slightly firm.

Meanwhile, peel and finely chop the onion. Wipe the mushrooms clean on kitchen paper, or quickly rinse them in a colander under a cold tap, and thinly slice them. Finely chop the garlic, or crush it in a mortar with a pestle.

Gently melt the butter in a large pan and add the walnuts. Cook them over a gentle heat for 2 - 3 minutes, stirring frequently, until they turn a deeper colour - taking care not to burn them.

Remove the walnuts and put them to one side. Wipe the pan clean and add the olive or walnut oil and onion to the pan. Cook over a moderate heat for 7-8 minutes, stirring occasionally, until the onion is soft and golden. Add the mushrooms and garlic, then turn to a high heat and cook for 5 minutes, stirring continuously, until the juices run and the mushrooms are tender.

Remove the pan from the heat. Add the walnuts and yoghurt, 1 tablespoon at a time, mixing well after each addition.

Return the pan to a very low heat and continue stirring until the yoghurt is heated through - taking care not to let the yoghurt boil and separate. Add half the parsley and season to taste with salt and freshly ground black pepper.

Drain the pasta quickly and immediately mix it with the sauce. Taste and adjust the seasoning. Transfer the pasta and its sauce to serving plates. Sprinkle over the remaining parsley and serve.